Composite electrode for electrochemical processing having improved high temperature properties and method for preparation by combustion synthesis

ABSTRACT

A composite electrode for electrochemical processing having improved high temperature properties, and a process for making the electrode by combustion synthesis. A composition from which the electrode is made by combustion synthesis comprises from about 4% to about 90% by weight of a particulate or fibrous combustible mixture which, when ignited, is capable of forming an interconnected network of a ceramic or metal-ceramic composite, and from about 10% to about 60% by weight of a particulate or fibrous filler material capable of providing the electrode with improved oxidation resistance and maintenance of adequate electrical conductivity at temperatures above 1000° C. The filler material is molybdenum silicide, silicon carbide, titanium carbide, boron carbide, boron nitride, zirconium boride, cerium oxide, cerium oxyfluoride, or mixtures thereof.

This is a divisional, of application Ser. No. 07/715,547, filed Jun. 14,1991, now allowed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an electrode for use in electrochemicalprocessing having improved oxidation and corrosion resistance incomparison to prior art electrodes used for the same purposes, which canbe readily produced by a process involving combustion synthesis to forma core body having an interconnected network of a ceramic ormetal-ceramic composite in which is uniformly dispersed a fillermaterial. Although not so limited the invention has particular utilityin the provision of an anode and a cathode for the electrowinning ofaluminum from its ore in the Hall-Herault process. The electrode of thepresent invention provides improved oxidation resistance at temperaturesabove 1000° C. with retention of satisfactory electrical and thermalconductivity at such elevated temperatures.

2. Description of the Prior Art

U.S. patent application Ser. No. 07/648,165, filed Jan. 30, 1991, in thenames of Jainagesh A. Sekhar and Sarit B. Bhaduri, and assigned to theassignee of the present application, discloses a composite electrode forelectrochemical processing and a method for preparation thereof bycombustion synthesis. Such an electrode comprises at least 20% by weightof a ceramic composite or a metal-ceramic composite in the form of adimensionally stable interconnected network, at least about 15% byweight of a filler material providing desired electrochemicalproperties, the filler material being uniformly dispersed in thecomposite network, and up to about 35% by weight of a binder phaseassociated with the network and the filler material. The ceramic ormetal-ceramic composite network is derived from a wide range ofcombustible mixtures which, when ignited, form the compositeinterconnected network or core. Filler materials are selected from avariety of nitrides, oxides, borides, carbides, silicides, oxyfluorides,phosphides, metals, and/or carbon. While the compositions and method ofpreparation of this application produce a dimensionally stable product,it has been found that electrodes made in accordance with the processare not stable above 1000° C.

"Encyclopedia Of Materials Science", Vol. 2, Michael B. Bever,editor-in-chief, Pergamon Press, 1986, page 1413, summarizes the stateof the art relating to electrode materials for electrochemicalprocessing, including electrochemical research, electrolytic productionof hydrogen, chlorine, chlorates, perchlorates, electrowinning ofaluminum, and other electrochemical processes. At page 1413, adiscussion of the electrometallurgy of aluminum points out thatelectrolysis of a cryolite-alumina (Na₃ AlF₆ +Al₂ O₃) melt is carriedout using a carbon anode and an aluminum cathode to yield aluminum onthe basis of the reaction:

    2Al.sub.2 O.sub.3 +3C→4Al+3CO.sub.2

Carbon dioxide is formed at the anode. The types of carbon anodepresently used are described, and it is also pointed out that carbon isused as a cell lining in the reduction cell. Lining failure and anodeconsumption are recognized as being major disadvantages in the presentprocess.

The use of combustion synthesis (CS), also referred to asself-propagating high-temperature synthesis (SHS), for a variety ofapplications is reviewed by H. C. Yi et al, in Journal MaterialsScience, 25, 1159-1168 (1990). It is concluded that almost all the knownceramic materials can be produced using the SHS method, in product formsincluding abrasives, cutting tools, polishing powders; elements forresistance heating furnaces; high temperature lubricants; neutronattenuators; shape-memory alloys; high temperature structural alloys;steel melting additives; and electrodes for electrolysis of corrosivemedia. It is acknowledged that considerable research is needed, andmajor disadvantages arise in "achieving high product density and tightcontrol over the reaction and products."

This article reports numerous materials produced by SHS and combustiontemperatures for some of them, viz., borides, carbides, carbonitrides,nitrides, silicides, hydrides, intermetallics, chalcogenides, cementedcarbides, and composites.

Combustion wave propagation rate and combustion temperature are statedto be dependent on stoichiometry of the reactants, pre-heatingtemperature, particle size, and amount of diluent.

J. W. McCauley et al, in "Simultaneous Preparation and Self-Centering OfMaterials In The System Ti-B-C", Ceramic Engineering and ScienceProceedings, 3, 538-554 (1982), describe SHS techniques using pressedpowder mixtures of titanium and boron; titanium, boron and titaniumboride; and titanium and boron carbide. Stoichiometric mixtures oftitanium and boron were reported to react almost explosively (wheninitiated by a sparking apparatus) to produce porous, exfoliatedstructures. Reaction temperatures were higher than 2200° C. Mixtures oftitanium, boron and titanium boride reacted in a much more controlledmanner, with the products also being very porous. Reactions of titaniumwith boron carbide produce material with much less porosity. Particlesize distribution of the titanium powder was found to have an importanteffect, as was the composition of the mixtures. Titanium particle sizesranging from about 1 to about 200 microns were used.

R. W. Rice et al, in "Effect Of Self-Propagating Synthesis ReactantCompact Character On Ignition, Propagation and ResultantMicrostructure", Ceramic Engineering and Science Proceedings, 7, 737-749(1986), describe SHS studies of reactions using titanium powders toproduce TiC, TiB₂ or TiC+TiB₂. Reactant powder compact density was foundto be a major factor in the rate of reaction propagation, with themaximum rate being at about 60±10% theoretical density. Reactantparticle size and shape were also reported to affect results, withtitanium particles of 200 microns, titanium flakes, foil or wire eitherfailing to ignite or exhibiting slower propagation rates. Particle sizedistribution of powder materials (Al, BC, Ti) ranged from 1 to 220microns.

U.S. Pat. No. 4,909,842, issued Mar. 20, 1990 to S. D. Dunmead et al,discloses production of dense, finely grained composite materialscomprising ceramic and metallic phases by SHS combined with mechanicalpressure applied during or immediately after the SHS reaction. Theceramic phase or phases may be carbides or borides of titanium,zirconium, hafnium, tantalum or niobium, silicon carbide, or boroncarbide. Intermetallic phases may be aluminides of nickel, titanium orcopper, titanium nickelides, titanium ferrides, or cobalt titanides.Metallic phases may include aluminum, copper, nickel, iron or cobalt.The final product is stated to have a density of at least about 95% ofthe theoretical density only when pressure is applied during firing, andcomprises generally spherically ceramic grains not greater than about 5microns in diameter in an intermetallic and/or metallic matrix.

U.S. Pat. No. 4,948,767, issued Aug. 14, 1990 to D. Darracq et al,discloses a ceramic/metal composite material, which may be used as anelectrode in a molten salt electrolysis cell for producing aluminum,having at least one ceramic phase and at least one metallic phase,wherein mixed oxides of cerium and at least one of aluminum, nickel,iron and copper are in the form of a skeleton of interconnected ceramicoxide grains, the skeleton being interwoven with a continuous metallicnetwork of an alloy or intermetallic compound of cerium with at leastone of aluminum, nickel, iron and copper. The ceramic phase may include"dopants" for increasing its electrical conductivity and/or density. Thedopants may comprise pentavalent elements such as tantalum and niobium,or rare earth metals. Inert reinforcing fibers or tissues may also bepresent. The method of production involves reactive sintering, reactivehot-pressing or reactive plasma spraying a precursor mixture containinga cerium oxide, fluoride and/or boride and/or at least one of aluminum,nickel, iron and copper. When used as an anode, the material is coatedwith a protective layer of cerium oxyfluoride. A significantdisadvantage of the process disclosed in this patent arises when theconstituents have widely different melting points, which makes sinteringor hot pressing into a dimensionally stable product impossible. Plasmaspray is a very limited technique which is unsuitable to form a largeanode or similar product within a reasonable time. It is also recognizedthat sintering of oxide and non-oxide materials is rarely possible, andthe interface bonding of materials by this technique may be inadequatefor acceptable mechanical and electrical properties.

Despite the recognition of the disadvantages of prior art electrodes andthe suggestion of the possibility of producing electrodes by CS, to thebest of applicant's knowledge there has been no successful applicationof CS techniques in the production of net shaped composite electrodesfor electrochemical processing which possess improved oxidationresistance while retaining adequate electrical conductivity attemperatures above 1000° C.

The Yi et al article acknowledged above does not recognize or suggestthe possibility of making composite electrodes by CS wherein desiredproperties are achieved by uniform dispersion of filler material in aceramic or metal-ceramic core body.

It is apparent that a need exists for an electrode suitable forelectrochemical processing, which exhibits improved oxidation andcorrosion resistance and retains satisfactory electrical conductivity attemperatures above 1000° C., and which avoids the disadvantages inherentin conventional electrodes.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a composition formaking an electrode suitable for electrochemical processing bycombustion synthesis, which will meet the above need.

It is a further object of the invention to provide a method of making anet shaped electrode suitable for electrochemical processing, bycombustion synthesis.

It is still another object of the invention to provide an electrode forelectrochemical processing which will meet the above need.

The above and other objects of the invention will be apparent from thedisclosure which follows.

According to the invention, there is provided a composition for makingan electrode suitable for electrochemical processing by combustionsynthesis, comprising from about 40% to about 90% by weight of aparticulate or fibrous combustible mixture which, when ignited, iscapable of forming an interconnecting network of a ceramic ormetal-ceramic composite, and from about 10% to about 60% by weight of aparticulate or fibrous filler material capable of providing saidelectrode with improved oxidation resistance and maintenance of adequateelectrical conductivity at temperatures above 1000° C., the fillermaterial being chosen from the group consisting of molybdenum silicide,silicon carbide, titanium carbide, boron carbide, boron nitride,zirconium boride, cerium oxide, cerium oxyfluoride, and mixturesthereof.

The term metal-ceramic composite is used herein to encompass alsointermetallic-ceramic composites which form an interconnected networkafter combustion synthesis.

The invention further provides a dimensionally stable electrode forelectrochemical processing made from the composition defined above.

The electrode of the invention may optionally contain up to about 5% byweight of a binder chosen from the group consisting of aluminum, nickel,copper, niobium, titanium, molybdenum, zirconium, rare earth metals,yttrium, and mixtures thereof, and a reinforcing material in fibrousform chosen from the group consisting of silicon carbide, graphite, ametal oxide, an elemental metal, a metal alloy, and mixtures thereof.

The invention further provides a method of making a net shaped electrodesuitable for electrochemical processing and having improved oxidationresistance and electrical conductivity at temperatures above 1000° C.,which comprises providing a uniform admixture containing from about 40%to about 90% by weight of a particulate or fibrous combustible mixturewhich, when ignited, is capable of forming an interconnecting network ofa ceramic or metal-ceramic composite, and from about 10% to about 60% byweight of a particulate or fibrous filler material chosen from the groupconsisting of molybdenum silicide, silicon carbide, titanium carbide,boron carbide, boron nitride, zirconium boride, cerium oxide, ceriumoxyfluoride, and mixtures thereof; compacting the admixture into a netshape in a die under a pressure of about 5 to about 25 ksi (about 3.5 toabout 17.6 kg/mm²); removing the net shape from the die; and ignitingthe combustible mixture whereby to obtain a dimensionally stablecomposite electrode.

The invention further provides an improvement in a process for producingmetallic aluminum by electrolysis of molten cryolite-alumina, byproviding non-consummable electrodes which minimize carbon consumptionand eliminate formation of carbon dioxide at the anode, the electrodescomprising from about 40% to about 90% by weight of a ceramic compositeor a metal-ceramic composite in the form of a dimensionally stableinterconnected network, and from about 10% to about 60% by weight of afiller material uniformly dispersed in the network, the filler materialbeing chosen from the group consisting of molybdenum silicide, siliconcarbide, titanium carbide, boron carbide, boron nitride, zirconiumboride, cerium oxide, cerium oxyfluoride, and mixtures thereof, theelectrodes exhibiting improved corrosion and oxidation resistance andmaintaining satisfactory electrical conductivity at temperatures above1000° C.

In the conventional process for electrolysis of molten cryolite-alumina,carbon is generally used as the reducing agent and is supplied both fromthe carbon anode and from the carbon lining in the reduction cell. Ifcarbon is used as the reducing agent in the method of the presentinvention, a carbon lining in the reduction cell would be needed as thecarbon source. However, consumption of the anode is eliminated in themethod of the invention along with the undesirable formation of carbondioxide at the anode. The overall consumption of carbon should thus beminimized. Moreover, the method of the invention could use a differentreducing agent, thus further minimizing or even eliminating carbonconsumption.

Electrodes in accordance with the invention may be used both as anodesand cathodes. The process for making such electrodes offers flexibilityin configuration since incorporation of cooling channels and a bipolarconfiguration of anodes is readily obtainable.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention will be described with particularreference to the improved high temperature oxidation and corrosionresistance and retention of electrical conductivity of electrodes havingspecific utility for electrowinning of aluminum. However, it should berecognized that other applications requiring such properties are withinthe scope of the invention.

The desired properties for electrodes for aluminum electrowinning arelow reactivity to molten cryolite in comparision to graphite;resistivity of 5-10 milliohm/cm; resistance to oxidation at temperaturesabove 1000° C.; and adequate electrical conductivity at temperaturesabove 1000° C.

Electrodes in accordance with the present invention exhibit the aboveproperties. Combustion synthesis is believed to provide the onlyeconomical way by which to make such electrodes. Moreover, in someinstances combustion synthesis is the only way of producing suchproducts, e.g., where the constituents have very different meltingpoints, making sintering by conventional techniques impossible.

In a preferred composition for making an electrode in accordance withthe invention the combustible mixture is chosen from the groupconsisting of:

from about 28% to about 32% titanium dioxide, about 25% to about 27%boron oxide, about 30% to about 35% aluminum, about 3% to about 4%titanium, about 1.5% to about 2% boron, about 4% to about 5% nickel, andabout 0.8% to about 1.0% phosphorus;

from about 65% to about 75% titanium, and about 25% to about 35% boron;

from about 60% to about 65% molybdenum, and about 35% to about 40%silicon;

from about 75% to about 85% titanium, and about 15% to about 25% carbon;

from about 40% to about 50% titanium, and about 50% to about 60% nickel;

from about 10% to about 20% aluminum, and about 80% to about 90% nickel;

from about 50% to about 55% molybdenum, about 30% to about 35% nickel,and about 15% to about 17% silicon;

from about 77% to about 80% boron, and about 20% to about 23% carbon;

from about 73% to about 85% zirconium, and about 15% to about 27% boron;and

mixtures thereof; all percentages being by weight of said combustiblemixture.

The filler material in a preferred composition comprises up to about 25%molybdenum silicide, up to about 18% silicon carbide, up to about 35%titanium carbide, up to about 25% boron carbide, up to about 25% boronnitride, up to about 50% zirconium boride, up to about 25% cerium oxide,and mixtures thereof, based on the total weight of the composition.

As indicated above, the composition may also include up to about 5% byweight of an inorganic binder having a melting point lower than thecombustion synthesis reaction temperature. Preferred binders include atleast one of aluminum, nickel, and copper.

All the starting components of the composition are in particulate orfibrous form. When in particulate form the components preferably have anaverage particle size of less than 44 microns (-325 mesh). Fibrousmaterial may have an average diameter of less than 44 microns and anaspect ratio of at least 2:1.

The method of the present invention for making a net shaped electrode issimilar to that disclosed in application Ser. No. 07/648,165acknowledged above. The disclosure of this copending application isincorporated herein by reference. In this method, a uniform mixture ofthe components is compacted into the desired net shape in a die under apressure of about 5 to about 25 ksi, preferably about 7 ksi (about 4.9kg/mm²). The net shape mixture is then removed from the die and ignitedby means of an electric arc, electric spark, flame, microwave, weldingelectrode, electron beam, laser or other conventional manner in order toinitiate combustion synthesis. Alternatively, the electrode may bepassed through an induction coil or furnace heated to the ignitiontemperature. If a binder is present, it melts during combustionsynthesis and becomes part of both the interconnected ceramic ormetal-ceramic network and the filler material.

After combustion synthesis the product in the form of a dimensionallystable electrode contains at least one of molybdenum silicide, siliconcarbide, nickel phosphide, titanium boride, titanium carbide, zirconiumboride, titanium nickel intermetallics, aluminum nickel intermetallics,and aluminum nickel-silicon-molybdenum intermetallics. Combustionsynthesis is believed to be the only way of making molybdenum silicidesat relatively low temperatures.

All the compositions specified herein form a very thin adherent oxidelayer on the surface of an electrode after being put in service. This isbelieved to be the reason for the high resistance to oxidation attemperatures above 1000° C. Moreover, stable electrical conductivity isretained at temperatures up to at least 1150° C.

It should be recognized that part or all of the combustible mixture mayfunction, after ignition, in the same manner as the filler material inproviding desired electrochemical properties. The binder, if present,may also function as a dopant for the ceramic composite after ignition.

A series of combustible mixtures were prepared and mixed in varyingproportions with filler materials to produce exemplary products bycombustion synthesis. Most of the starting materials were in particulateform with average particle sizes of less than 44 microns, i.e., passing325 mesh screen. Nickel powder, when used, ranged from 3 to 100 micronsin average particle size, with 3 microns being preferred. The componentswere mixed uniformly and compacted under pressures ranging from about 5to about 25 ksi into net shapes suitable for testing.

The compositions of combustible mixtures, in weight percent, were asfollows:

    ______________________________________                                        Combustible Mixtures-Weight Percent                                           ______________________________________                                        Composition I                                                                         TiO.sub.2   30.00%                                                            B.sub.2 O.sub.3                                                                           26.25%                                                            Al          33.75%                                                            Ti          3.25%                                                             B           1.75%                                                             Ni          4.10%                                                             P           0.90%                                                     Composition II                                                                        Ti          70%                                                               B           30%                                                       Composition III                                                                       Mo          63%                                                               Si          37%                                                       Composition IV                                                                        Ti          80%                                                               C           20%                                                       Composition V                                                                         Ti          45%                                                               Ni          55%                                                       Composition VI                                                                        Al          15-20%                                                            Ni          80-85%                                                    Composition VII                                                                       Mo          52.5%                                                             Ni          32.1%                                                             Si          15.4%                                                     Composition VIII                                                                      B           78.3%                                                             C           21.7%                                                     Composition IX                                                                        Zr          75%                                                               B           25%                                                       ______________________________________                                    

Exemplary compositions utilizing various proportions of the abovecombustible mixtures were then prepared by uniform admixture with fillermaterials, compacted in a die, removed from the die, and ignited to formnet shaped test specimens. These examples were as follows:

    ______________________________________                                        Example 1                                                                            Comp. I        16.67%                                                         Comp. II       29.16%                                                         SiC            16.67%                                                         MoSi.sub.2     25.00%                                                         CeO.sub.2      12.50%                                                  Example 2                                                                            Comp. I        40%                                                            Comp. III      40%                                                            SiC            10%                                                            CeO.sub.2      10%                                                     Example 3                                                                            Comp. I        5%                                                             Comp. II       25%                                                            Comp. III      40%                                                            SiC            10%                                                            CeO.sub.2      15%                                                            Ni (binder)    5%                                                      Example 4                                                                            Comp. III      40%                                                            TiC            20%                                                            SiC            15%                                                            CeO.sub.2      25%                                                     Example 5                                                                            Comp. III      35%                                                            TiC            25%                                                            SiC            15%                                                            CeO.sub.2      20%                                                            Ni (binder)    5%                                                      Example 6                                                                            Comp. II       5%                                                             Comp. III      35%                                                            TiC            25%                                                            SiC            10%                                                            CeO.sub.2      20%                                                            Ni (binder)    5%                                                      Example 7                                                                            Comp. III      40%                                                            TiC            35%                                                            SiC            10%                                                            CeO.sub.2      15%                                                     Example 8                                                                            Comp. III      35%                                                            Comp. V        20%                                                            TiC            10%                                                            SiC            10%                                                            CeO.sub.2      15%                                                            MoSi.sub.2     10%                                                     Example 9                                                                            Comp. III      35%                                                            Comp. V        30%                                                            SiC            10%                                                            CeO.sub.2      15%                                                            MoSi.sub.2     10%                                                     Example 10                                                                            Comp. III     30%                                                            Comp. V        20%                                                            TiC            10%                                                            SiC            10%                                                            CeO.sub.2      15%                                                            MoSi.sub.2     10%                                                            Ni (binder)    5%                                                      Example 11                                                                           Comp. II       10%                                                            Comp. III      30%                                                            Comp. V        45%                                                            SiC            15%                                                     Example 12                                                                           Comp. III      40%                                                            Comp. V        40%                                                            SiC            10%                                                            MoSi.sub.2     10%                                                     Example 13                                                                           Comp. II       10%                                                            Comp. III      30%                                                            Comp. V        37.5%                                                          SiC            17.5%                                                          Al (binder)    5%                                                      Example 14                                                                           Comp. III      50%                                                            Comp. V        30%                                                            SiC            10%                                                            MoSi.sub.2     10%                                                     Example 15                                                                           Comp. III      30%                                                            Comp. V        50%                                                            SiC            10%                                                            MoSi.sub.2     10%                                                     Example 16                                                                           Comp. III      10%                                                            Comp. VI       80%                                                            SiC            5%                                                             MoSi.sub.2     5%                                                      Example 17                                                                           Comp. VI       90%                                                            SiC            5%                                                             MoSi.sub.2     5%                                                      Example 18                                                                           Comp. VI       80%                                                            SiC            10%                                                            MoSi.sub.2     10%                                                     Example 19                                                                           Comp. VI       75%                                                            SiC            10%                                                            MoCi.sub.2     10%                                                            Al (binder)    5%                                                      Example 20                                                                           Comp. III      40%                                                            Comp. VI       50%                                                            SiC            5%                                                             MoSi.sub.2     5%                                                      Example 21                                                                           Comp. III      45%                                                            Comp. VI       45%                                                            SiC            5%                                                             MoSi.sub.2     5%                                                      Example 22                                                                           Comp. III      45%                                                            Comp. VI       40%                                                            SiC            5%                                                             MoSi.sub.2     5%                                                             CeO.sub.2      5%                                                      Example 23                                                                           Comp. VI       70%                                                            SiC            5%                                                             MoSi.sub.2     10%                                                            CeO.sub.2      10%                                                            Al (binder)    5%                                                      Example 24                                                                           Comp. VI       45%                                                            SiC            10%                                                            MoSi.sub.2     20%                                                            CeO.sub.2      20%                                                            Al (binder)    5%                                                      Example 25                                                                           Mo             52.5%                                                          Ni             32.1%                                                          Si             15.4%                                                   Example 26                                                                           Comp. VI       75%                                                            B.sub.4 C      25%                                                     Example 27                                                                           Comp. VI       30%                                                            Comp. VII      45%                                                            B.sub.4 C      25%                                                     Example 28                                                                           Comp. VI       30%                                                            Comp. VII      45%                                                            B.sub.4 C      15%                                                            CeO.sub.2      10%                                                     Example 29                                                                           Comp. VI       70%                                                            Comp. VIII     15%                                                            B.sub.4 C      10%                                                            CeO.sub.2      2.5%                                                           Ti (binder)    2.5%                                                    Example 30                                                                           Comp. VI       30%                                                            Comp. VII      45%                                                            Comp. VIII     7.5%                                                           B.sub.4 C      10%                                                            CeO.sub.2      5%                                                             Ti (binder)    2.5%                                                    Example 31                                                                           Comp. III      45%                                                            Comp. VI       45%                                                            SiC            5%                                                             MoSi.sub.2     5%                                                      Example 32                                                                           Comp. VI       38.0%                                                          Comp. VII      42.8%                                                          B.sub.4 C      4.8%                                                           CeO.sub.2      4.8%                                                           MoSi.sub.2     4.8%                                                           SiC            4.8%                                                    Example 33                                                                           Comp. III      45%                                                            Comp. VI       45%                                                            SiC            5%                                                             CeO.sub.2      5%                                                      Example 34                                                                           Comp. III      45%                                                            Comp. VI       40%                                                            SiC            5%                                                             CeO.sub.2      4%                                                             MoSi.sub.2     5%                                                             Nb (binder)    1%                                                      Example 35                                                                           Comp. VII      30%                                                            Comp. IX       20%                                                            ZrB.sub.2      50%                                                     ______________________________________                                    

Example 25 is illustrative of the concept of utilizing the combustiblemixture (Composition VII), after ignition, to function as the fillermaterial, i.e., an in situ formation of filler material.

Test specimens were prepared from all specific examples, each havingdimensions of 3 mm×3 mm×10 mm. All specimens were tested for signs ofcatastrophic oxidation and instability by heating in air at 1050° C. for16 hours. All specimens were found to have good oxidation resistance bythis test. The greatest change in dimensions for any specimen was about2%.

Test specimens from Examples 25, 32 and 34 were also tested in aluminumelectrode cells with molten cryolite. Example 25 was run for 4 hours,and no change in dimensions was noted. Example 32 lasted for 6.25 hours,after which the tip of the electrode was lost in cryolite. This was animprovement over the normal life of a similar size graphite electrode.Example 34 was run for 3 hours (approximately the life of a similar sizeuncoated graphite electrode). The surface was noted as starting todeteriorate, but the specimen was intact.

The above tests in cryolite demonstrate superiority over conventionalgraphite electrodes.

Electrical resistivity was tested in specimens from Example 21.Resistivity remained unchanged after exposure to air for 24 days at1050° C.

A specimen was prepared with the composition of Example 1, having acopper wire inserted through the center of the matrix. Combustionsynthesis of this specimen was successful, and the resultingmetal-ceramic composite adhered strongly to the copper wire surface.Accordingly, cathodes and anodes could be prepared, in accordance withthe invention, in the form of a surface layer of the combustionsynthesis product and filler material over a conductive core material.This would reduce the cost of such products.

Mechanical properties of all specimens of the specific examples wereacceptable. In this connection, it is considered that electrodes need becapable only of supporting their own weight.

I claim:
 1. A combustion synthesis product manufactured usingcomposition comprising from about 40% to about 90% by weight of aparticulate or fibrous combustible mixture which, when ignited, iscapable of forming an interconnecting network of a ceramic ormetal-ceramic composite, said composite being designed to cause theformation of an adherent oxide layer on the surface of said productduring said electrochemical processing, and from about 10% to about 60%by weight of a particulate or fibrous filler material capable ofproviding said electrode with improved oxidation resistance andmaintenance of adequate electrical conductivity at temperatures above1000° C., said filler material being chosen from the group consisting ofmolybdenum silicide, silicon carbide, titanium carbide, boron carbide,boron nitride, zirconium boride, cerium oxide, cerium oxyfluoride, andmixtures thereof, said filler material being chosen so as to providesaid electrode with improved oxidation resistance and thermal andelectrical conductivity above 1000° C.; andsaid product having anadherent oxide layer on the surface thereof, said layer being formedafter said product is put into service, said oxide layer lending to saidproduct, a high resistance to oxidation and stable electrical andthermal conductivity at temperatures above 1000° C.
 2. The product ofclaim 1, comprising from about 40% to about 85% of said combustiblemixture, from about 10% to about 55% of said filler material, and up toabout 5% by weight of a particulate or fibrous inorganic binder having amelting point lower than the combustion synthesis reaction temperature.3. The product of claim 1, wherein said combustible mixture is chosenfrom the group consisting of:from about 28% to about 32% titaniumdioxide, about 25% to about 27% boron oxide, about 30% to about 35%aluminum, about 3% to about 4% titanium, about 1.5% to about 2% boron,about 4% to about 5% nickel, and about 0.8% to about 1.0% phosphorus;from about 65% to about 75% titanium, and about 25% to about 35% boron;from about 60% to about 65% molybdenum, and about 35% to about 40%silicon; from about 75% to about 85% titanium, and about 15% to about25% carbon; from about 40% to about 50% titanium, and about 50% to about60% nickel; from about 10% to about 20% aluminum, and about 80% to about90% nickel; and from about 50% to about 55% molybdenum, about 30% toabout 35% nickel, and about 15% to about 17% silicon; from about 77% toabout 80% boron, and about 20% to about 23% carbon; from about 73% to85% zirconium, and about 15% to about 27% boron; and mixtures thereof;all percentages being by weight of said combustible mixture.
 4. Theproduct of claim 1, wherein said filler material comprises up to about25% molybdenum silicide, up to about 18% silicon carbide, up to about35% titanium carbide, up to about 25% boron carbide, up to about 25%boron nitride, up to about 0% zirconium boride, up to about 25% ceriumoxide, and mixtures thereof, based on the total weight of saidcomposition.
 5. The product of claim 1, wherein said combustible mixtureand said filler material have an average particle size of less than 44microns (-325 mesh).
 6. The product of claim 2, wherein said binder, isat least one of aluminum, nickel, copper, niobium, titanium, molybdenum,zirconium, rare earth metals, and yttrium.
 7. A dimensionally stableelectrode for electrochemical processing having improved oxidationresistance, electrical conductivity and thermal conductivity attemperatures above 1000° C., said electrode comprising:from about 40% toabout 90% by weight of a ceramic composite or a metal-ceramic compositein the form of a dimensionally stable interconnected network, saidcomposite being designed to cause the formation of an adherent oxidelayer on the surface of said electrode during said electrochemicalprocessing, and from about 10% to about 60% by weight of a fillermaterial uniformly dispersed in said network, said filler material beingchosen from the group consisting of molybdenum silicide, siliconcarbide, titanium carbide, boron carbide, boron nitride, zirconiumboride, cerium oxide, cerium oxyfluoride, and mixtures thereof, saidfiller material being chosen so as to provide said electrode withimproved oxidation resistance and thermal and electrical conductivityabove 1000° C.; and an adherent oxide layer on the surface thereof, saidlayer being formed after said electrode is put into service, said oxidelayer lending to said electrode, said high resistance to said oxidationand said stable electrical and thermal conductivity at temperaturesabove 1000° C.
 8. The electrode of claim 7, wherein said filler materialcomprises up to about 25% molybdenum silicide, up to about 18% siliconcarbide, up to about 35% titanium carbide, up to about 25% boroncarbide, up to about 25% boron nitride, up to about 50% zirconiumboride, up to about 25% cerium oxide and mixtures thereof, based on thetotal weight of the electrode.
 9. The electrode of claim 7, comprisingfrom about 40% to about 85% of said ceramic composite or metal-ceramiccomposite, from about 10% to about 55% of said filler material, and upto about 5% by weight of a metallic binder chosen from the groupconsisting of aluminum, nickel, copper, niobium, titanium, molybdenum,zirconium, rare earth metals, yttrium, and mixtures thereof.
 10. Theelectrode of claim 7, including a reinforcing material in fibrous formchosen from the group consisting of silicon carbide, graphite, a metaloxide, an elemental metal, a metal alloy, and mixtures thereof.
 11. Theelectrode of claim 7 in the form of a net shape anode, for use in theelectrowinning of aluminum from its oxides.
 12. The electrode of claim 7in the form of a net shape cathode, for use in the electrowinning ofaluminum from its oxide.
 13. An electrolytic cell for use in theelectrowinning of aluminum from its oxide containing at least oneelectrode in accordance with claim
 7. 14. A cathode for use in theelectrowinning of aluminum from its oxide, wherein a surface layer ofsaid cathode comprises the combustion synthesis product-of claim
 1. 15.In a process for producing metallic aluminum by electrolysis of moltencryolite-alumina, the improvement which comprises providingnonconsumable electrodes which minimize carbon consumption and eliminateformation of carbon dioxide at the anode, said electrodes comprisingfrom about 40% to about 90% by weight of a ceramic composite or ametal-ceramic composite in the form of a dimensionally stableinterconnected network, said composite being designed to cause theformation of an adherent the surface of said product during saideletrochemical processing, and from about 10% to about 60% by weight ofa filler material uniformly dispersed in said network, said fillermaterial being chosen from the group consisting of molybdenum silicide,silicon carbide, titanium carbide, boron carbide, boron nitride,zirconium boride, cerium oxide, cerium oxyfluoride, and mixturesthereof, said filler material being chosen so as to provide saidelectrode with improved oxidation resistance and thermal and electricalconductivity above 1000° C.; andsaid electrodes having an adherent oxidelayer on the surface thereof, said layer being formed after saidelectrode is put into service, said oxide layer lending to saidelectrode, said high resistance to said oxidation and said stableelectrical conductivity at temperatures above 1000° C.
 16. Theimprovement of claim 15, wherein said electrodes have cooling channelstherein and are arranged in a bipolar configuration.
 17. The improvementof claim 15, wherein said filler material comprises up to about 25%molybdenum silicide, up to about 18% silicon carbide, up to about 35%titanium carbide, up to about 25% boron carbide, up to about 25% boronnitride, up to about 50% zirconium boride, up to about 25% cerium oxide,and mixtures thereof, based on the total weight of said electrodes.